The invention relates to a method for determining a visual range from the backscatter signal of a light pulse emitted by a light source. In this method, a small part of the emitted light signal is coupled-in directly into a receiver for the normalization of the backscatter signal with the intensity of the emitted light pulse. Furthermore, in the receiver, the intensity of the backscatter signal is measured as a function of the transit time which has elapsed since the emission of the light pulse. A conclusion is drawn from the transit time of the backscatter signal as to the distance from a backscatterer.
Such a method is disclosed, by way of example, in German Patent document DE 4,005,919 C1. In the known German Patent document, it is proposed, for the measurement of visual range, to arrange in the front region of a vehicle, an emitter which is pivotable about a horizontal axis and which at regular time intervals emits light pulses directed onto the roadway at differing angles of inclination. The reflected, backscattered light pulses are received in a receiver arrangement. In this case, a plurality of receivers are disposed at respectively different angles of inclination in relation to the horizontal.
The signals emitted from the emitter pass onto the surface of the road at a distance which is essentially dependent upon the angle of inclination. The signals are reflected by the road surface and the reflected signal is measured in the appropriately inclined receiver.
A visual range measurement requires a plurality of individual measurements which in each instance only establishes whether it is or is not possible to see to a specified distance. Only all measurements together permit a statement to be made concerning the actual visual range. Furthermore, either an emitter must be disposed to be pivotable and controlled, which emitter emits the signal in a precisely predetermined direction, or it is necessary to dispose a plurality of individually driven, mutually inclined emitters. In any event, it is necessary to dispose a large number of receivers on the vehicle, to permit a determination to be made of the angle at which the reflected signals are incident, in relation to the horizontal.
In this manner, where an obstacle is situated within the visual range, it is not possible to determine the visual range but at best only the distance of the obstacle from the vehicle. It is to be seen as a further disadvantage that this distance measurement gives useful measurement values only on straight portions of a path of travel.
The object of the invention is to improve the visual range measurement adopted as a basis according to the relevant category to the end that the measurement of the visual range can take place using one emitter and using one receiver. The emitter and receiver are stationarily disposed. The intention is also to permit the determination of the visual range, even when an obstacle is situated within the field of view.
Adopting as a basis the visual range measurement according to the relevant category, this object is achieved according to the invention by a method for determining the visual range from the backscatter signal of a light pulse emitted by a light source. In this method, a small part of the emitted light signal is coupled-in directly into a receiver for the normalization of the backscatter signal with the intensity of the emitted light pulse. Furthermore, in the receiver, the intensity of the backscatter signal is measured as a function of the transit time which has elapsed since the emission of the light pulse. A conclusion is drawn from the transit time of the backscatter signal as to the distance from a backscatterer. A specimen curve, dependent upon at least two parameters of the intensity progression is adapted to the measured backscatter signal by determination of the parameters. At least one of the parameters is employed as a measure of the visual range.
The comparison with a specimen curve permits the determination of the visual range from the intensity curve obtained, without any need, for this purpose, of further information such as for example the direction from which the backscatter signal comes. It thus also becomes possible to determine a value for the visual range from each emitted pulse, so that it is also no longer necessary to evaluate a plurality of measurement results in order to obtain an indication of the visual range. As a result of this, it is also possible to attain a higher measurement frequency, since the duration of an emitted pulse does not need to be any greater than is the case when using the known arrangement. The method is suitable both for the stationary measurement of the visual range and also for use in a vehicle.
By a dual evaluation of the backscatter signal obtained from a measurement, it is moreover possible in an embodiment of the invention to also determine the visual range in circumstances in which an obstacle is situated within the field of view. After a first evaluation, in which the parameters are first determined, the measured intensity distribution is investigated for a limited region of great increase of the measured intensity from the specimen curve. In the event that such a region is present, this signal is interpreted as backscatter by an obstacle. The distance of the obstacle from the vehicle is then obtained from the transit time of the increased signal. To determine the visual range, a second computation of the parameters is carried out. In this case, the region of the signal increase is left out of consideration in the computation of the parameters.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.